Several ways to reduce the chances of forging documents by methods, such as photocopying, already exist in the prior art. For example U.S. Pat. Nos. 4,227,720 and 4,310,180 disclose a system for protecting photolithographically prepared documents which employs a masked warning mark that is said to clearly appear on copies due to the inability of color copiers to integrate a composite pattern of big and little dots. On the original document, the mark is at least partially concealed from the casual observer. The system utilizes a mask having small dots of color density below the color reproductive density of the copier, while the overlay of the mask and warning phrase has larger dots of color density that exceeded the color reproductive density of the copier. Depending on the quality of the printing and the copier, it has been observed that in some cases the warning mark does not always clearly appear when a document is photocopied.
Also well-known in the prior art are methods for accurately printing very fine lines and other elements such as by intaglio printing, which is generally illustrated in FIG. 1 and discussed further below. Intaglio printing has been previously used to imprint security images into documents of value as disclosed in U.S. Pat. No. 4,033,059. In that patent, the pattern elements defining the image portions differ in depth or orientation from those elements forming the background. The object is to make a document in which the ability to discern the image portion from the background varies noticeably depending upon the angle of view and the orientation of the document, a characteristic not passed on to copies of the document. Both latent and transient images can be used to achieve this object. As viewed from a direction normal to the document surface, the latent image blends visually with the background. However, when the document is viewed at an acute angle to its surface, the latent image is readily recognizable in contrast to the background. The transient image is discernable when the document is viewed from a direction normal to its surface but disappears as the angle of view becomes acute. Whether a transient or latent image is used, copies of the document will not have the characteristic of a changing relationship between the contrast of the image portion with respect to the background as a function of changing angle of view. Conversely, as shown in U.S. Pat. No. 1,002,600, it is also known to provide distinctive marks which consist of lined elements produced at angles to the lines of the ground-work, which marks are "invisible" except when inspected through a special detector.
Our co-pending application, entitled, "Intaglio Printing Method and Secure Document Having a Variable Optical Image," provides a further improvement on the method of intaglio printing for secure documents. In that application, a composite design is engraved into an intaglio printing plate which is mounted on a roller and then coated with reflective intaglio ink and then pressed into a document substrate. Depending on the angle of view of the observer, one design of the composite design will reflect light and be immediately apparent, while the other design will blend substantially with the substrate surface. A photocopy or facsimile of the design will lack these changing reflective qualities.